1. Field of the Invention
The present invention relates to improvement of Ti-Al alloys, particularly, alloys in which the main constituent phase is the intermetallic compound, TiAl.
2. Prior Art
Machine parts which are used under rotary or reciprocal movement, for example, turbine blades, hot wheels of turbochargers and engine valves, are recently being more and more light-weighted in order to meet the requirements of high performance such as high responce and high output. Heat-resistant materials for the above noted parts are, therefore evaluated by their specific strength (strength/density) rather than the absolute strength, and efforts are being made to improve the specific strength of these materials.
Under the circumstances, Ti-Al alloys, particularly, alloys in which the main constituent phase is intermetallic compound, TiAl, are drawing attention. The maximum usable temperature (a temperature at which the creep rupture life is 1000 hours under stress of 28.1 Kgf/mm.sup.2) of TiAl is 800.degree. C., which is higher than that of conventional titanium alloy (Ti-6Al-4V), 550.degree. C. Moreover, the specific gravity of TiAl (3.8) is lower than that of the conventional titanium alloy (4.5) and is closer to that of ceramics (e.g., Si.sub.3 N.sub.4 3.2). TiAl has a ductility which ceramics lack, and its specific strength is higher than that of nickel-based super-alloys (e.g., Inconel 713C).
Ti-AL alloys in which the main constituent phase is TiAl, however, have lower ductility when compared with the titanium alloys and nickel-based super-alloys, and have the drawback of poor plastic workability. Efforts are being made to improve this (for example, Japanese Patent Disclosure 56-4344 discloses addition of appropriate amount of V), but have not yet reached practical use. Also, the melting point of the intermetallic compound, TiAl, exceeds 1500.degree. C. which is higher than those of the nickel-based super-alloys for casting use (usually, 1250-1400.degree. C.), and therefore, it is difficult to obtain defectless cast products having desired shape by conventional lost-wax method using ceramic molds due to chemical reactions between the active molten metal, TiAl, of a high temperature exceeding 1500.degree. C. and ceramics forming the molds.